1. Field of the Invention
The present invention relates to a transmitter and a receiver which conduct communication with each other according to a spread spectrum system, and in particular, to a transmitter which is provided in a base station and transmits a signal to a user who accomplishes a high-speed transmission via an antenna having a narrow directivity to minimize interference in other users, and to a receiver which is provided in a mobile station and in which a signal received from a sector antenna and a signal received from an antenna having a narrow directivity are both demodulated to continuously conduct communication even when there is interference by a user conducting a high-speed communication.
2. Description of the Related Art
In a code division multiple access (CDMA) communication system according to a spread spectrum system for communication of mobile stations such as car telephones and hand-held telephones, in order to support a large number of users, a service area is subdivided into a plurality of cells and each cell is further divided into a plurality of sectors. For example, one cell is subdivided into three sectors, as shown in FIG. 1. Since there are used a radio wave of the same carrier frequency and the same spreading code in each sector, the number of users to be supported can be three-times that of the users in the system in which the cell division is not adopted. For these three sectors, there are utilized first to third sector antennas 1 to 3 each having a wide directivity, each antenna being capable of conducting communication in a fan-shaped area having a central angle of 120 degrees. As shown in FIG. 2, a transmission signal to a first user in the area (first sector) of the first sector antenna 1 is quadrature-modulated in a first modulator 9 and is subjected to a frequency conversion and a power amplification in a first transmission circuit 5 to be transmitted via the first sector antenna 1. A transmission signal to a second user in the area (second sector) of the second sector antenna 2 is quadrature-modulated in a second modulator 10 and is subjected to a frequency conversion and a power amplification in a second transmission circuit 6 to be transmitted via the second sector antenna 2. Similarly, a transmission signal to a third user in the area (third sector) of the third sector antenna 3 is quadrature-modulated in a third modulator 11 and is subjected to a frequency conversion and a power amplification in a third transmission circuit 7 to be transmitted via the third sector antenna 3. Additionally, for example, when a plurality of users exist in one sector, the transmission signals to the respective users are spread with mutually different spreading codes. The resultant signals are combined with each other to be transmitted from the first sector antenna 1.
In the CDMA communication system, with the length of spreading codes kept unchanged, the communication speed can be varied by increasing or decreasing a one-symbol period of time. That is, when the one-symbol period is long, the cycle of spreading code is elongated; conversely, when the one-symbol period is short, the cycle of spreading codes is reduced. When a spreading code with a long cycle is used, the number of chips received during the one-symbol period becomes greater and hence the communication speed is lowered; however, since the reception signals can be integrated for a long period of time, the transmission power is minimized. On the other hand, when a spreading code with a short cycle is used, the number of chips received during the one-symbol period is decreased and hence the communication speed becomes higher; however, the transmission power is increased.
The low-speed communication using a spreading code having a long cycle is suitable for a voice communication or the like. Moreover, since the communication is achieved with low power and does not disturb the other users through interference, there is attained an advantage that the quality of communication is not lowered even when a large number of users conduct communication at the same time. On the other hand, the high-speed communication using a spreading code having a short cycle is suitable for the multimedia communication such as an image communication. However, since the communication is achieved with high power and considerably interferes with other users, there arises a problem that the communication can be conducted only between a small number of users. For example, even if four low-speed communication users can be supported in one sector, only one high-speed communication user can be supported. Consequently, when the high-speed communication user is in communication, the other low-speed communication users cannot conduct communication in the pertinent sector.
In order to support a high-speed communication user even when many low-speed communication users are conducting communication, the cell is conventionally subdivided into a larger number of sectors (e.g. 12 sectors) such that the high-speed communication user does not interfere with the low-speed communication users in other sectors.
However, in the conventional method, since the number of sectors is increased to minimize the amount of interference in low-speed communication users in other sectors due to the high-speed communication user, there appears a problem that the large number of sectors are to be controlled by the base station and hence the sector control operation is complicated. Furthermore, hen the sector becomes smaller, the number of hand-over operations is increased between sectors. This consequently results in a problem, namely, to achieve the hand-over operations, it is necessary to transmit the same signal to a plurality of sectors and hence the transmission power becomes higher and the amount of interference in other users is increased.
For a mobile station, since there are used quite small sectors, even when the moving distance thereof is short, the chance to make a search for a hand-over destination occurs more frequently. This leads to a problem that the hardware size becomes greater and the consumption power is increased.